1. Field of the Invention.
This invention pertains to internal combustion engines, and more particularly to apparatus for conducting exhaust products from an engine cylinder to the atmosphere.
2. Description of the Prior Art.
For two reasons, the design of internal combustion engines requires that much attention be paid to their exhaust systems. First, especially for two-stroke engines, the performance of an engine is greatly affected by the exhaust system. Second, the exhaust from an engine is very noisy and must be silenced. To some degree, the desirable characteristics of high performance and low noise are incompatible with each other.
Exhaust systems for internal combustion engines are very complicated, both because of the dynamics of the gasses and because of space constraints. Many small two-stroke industrial engines have severe limitations on the amount of space available for the exhaust systems. In those cases, the available space is usually taken up almost entirely by silencing components, often to the detriment of engine performance.
On the other hand, high performance engines such as are used on motorcycles and snowmobiles usually have more space available for the exhaust systems. Even with high performance engines, however, it is a difficult task to design a performance enhancing, quiet, and aesthetically pleasing exhaust system.
It is well known that pressure waves of the exhaust gasses reflect at a change in cross section of an exhaust pipe. It is further known to take advantage of the pressure wave reflections to tune the exhaust system in a manner that enhances engine performance. Particularly, exhaust systems are designed such that exhaust pulses from the engine are reflected cyclically back to the engine exhaust port. For a portion of each cycle, the exhaust pulse is reflected as an expansion wave that assists in scavaging the engine cylinder. For another portion of the cycle, the exhaust pulse is reflected back to the exhaust port as a compression wave that serves to hold the fresh charge in the cylinder.
Another complicating factor in the design of exhaust systems is that a particular design is optimum for only a narrow range of operating speeds. Accordingly, an exhaust system that is tuned for a particular engine and speed will not be the optimum system for the same engine at a different speed. To date, size and space limitations have made it impractical to equip an engine with an exhaust system that is optimum for more than one operating speed.
The design of two-stroke engines is discussed at length in the books "The Basic Design of Two-Stroke Engines" and "Design and Simulation of Two-Stroke Engines", both by Gordon P. Blair and published by the Society of Automotive Engineers in 1990 and 1996, respectively. Both books include the scientific and engineering aspects of two-stroke engines, including their exhaust systems.
The book "Four-Stroke Performance Tuning" by A. Graham Bell, Haynes Publishing, copyright 1991, includes a chapter on exhaust systems. V. P. Engineering Inc. & Audie Technology Inc. have developed computer programs that simulate exhaust dynamics in four-stroke engines. One such program is entitled "Dynomation Four Cycle Wave Action Simulator," copyright 1994.
A common characteristic of all known literature on the topic of internal combustion engine exhaust systems is that the cross sections of the exhaust passages are described as being round for their full lengths. For example, the above-mentioned book by Bell gives formulas for the size of exhaust pipes that are based on pipe diameters. In practice, because of manufacturing limitations, the cross sectional shapes of some exhaust pipes had been slightly elliptical in cross section. Although elliptical exhaust passages were sometimes tolerated by engine manufacturers, the consensus was that they compromised overall engine performance.
To tune an exhaust system for performance, the desired engine speed was chosen. An exhaust pipe was designed that had discrete sections of different lengths and diameters chosen to suit the particular engine and operating speed. The diameters of the various sections may be constant or variable along their lengths, i.e., the sections may be cylindrical or conical. FIGS. 1 and 2 show an ideal prior tuned exhaust pipe 1 for a single-cylinder four-stroke engine 2. The exhaust pipe 1 is duplicated for each cylinder of a multi-cylinder engine. The exhaust pipe has a header 3 and a megaphone section 5, both of which are circular in cross section and symmetrical about a straight longitudinal axis 7. Exhaust pulses from the engine 2 are reflected back as expansion waves in the megaphone section 5.
FIGS. 3 and 4 show a typical prior exhaust pipe 9 tuned for performance for a two-stroke engine 10. The expansion pipe 9 has a header 11 at the inlet end and a stinger 12 at the outlet end. Between the header 11 and the stinger 12 are intermediate sections 13, 14, 15, 16, and 17. In the particular exhaust 9 pipe shown, pulses from the engine 10 are reflected back as expansion waves within the sections 13, 14, and 15, and as compression waves in the section 17.
In practice, the ideal exhaust pipes 1 and 9 can rarely, if ever, be used. That is because space considerations on the vehicle propelled by the engine, such as a snowmobile or motorcycle, usually dictate that the pipe be curved in some fashion, often in a U-shape. U.S. Pat. No. 4,285,109 describes a method suitable for manufacturing a curved exhaust pipe.
Another factor that complicates the design of engine exhaust systems is the fact that most engines have more than one cylinder and consequently more than one exhaust pipe. The problem of finding space on a vehicle for multiple exhaust pipes can be difficult to solve, particularly when aesthetics are important. The envelope of multiple properly tuned exhaust pipes occupies a significant amount of space on the vehicle. FIGS. 5 and 6 show a typical prior tuned exhaust system 19 for a three-cylinder two-stroke engine. It will be appreciated that there is considerable wasted space within the three-lobe envelope 18 of the three individual exhaust pipes 22 of the system 19.